The melanocortin-4 receptor (MC4R) is a critical regulator of food intake and energy homeostasis (Cone R D (2005) Anatomy and regulation of the central melanocortin system. Nature Neuroscience 8(5):571-578). Furthermore, up to 5% of cases of severe pediatric obesity are due to heterozygosity for deleterious mutations in the MC4R gene (Farooqi I S, et al. (2003) Clinical spectrum of obesity and mutations in the melanocortin 4 receptor gene. N Engl J Med 348(12): 1085-1095).
Consequently, the MC4R has been considered a high priority target for the development of potent and selective orthosteric agonists for the treatment of obesity. Several groups have been successful at developing such compounds, including peptides and small molecules, and these molecules showed efficacy in significantly reducing weight in animal models from mice to primates (Kievit P, et al. (2012) Chronic treatment with a melanocortin-4 receptor agonist causes weight loss, reduces insulin resistance, and improves cardiovascular function in diet-induced obese rhesus macaques. Diabetes 62(2):490-497; Hess S, et al. (2008) Backbone cyclic peptidomimetic melanocortin-4 receptor agonist as a novel orally administrated drug lead for treating obesity. J Med Chem 51(4):1026-1034; Joseph C G, et al. (2008) The 1,4-benzodiazepine-2,5-dione small molecule template results in melanocortin receptor agonists with nanomolar potencies. J Med Chem 51(5):1423-1431.
Unfortunately, most MC4R agonists tested caused an unacceptable target-mediated pressor effect that caused the termination of the trials (Greenfield J R, et al. (2009) Modulation of blood pressure by central melanocortinergic pathways. N Engl J Med 360(1):44-52). Because the MC4R is a critical regulator of energy homeostasis, it remains a very valuable target for the treatment of obesity, assuming molecules could be developed that segregate the weight loss and pressor activities. Indeed, two recently described peptide analogues of the endogenous MC4R ligand, α-melanocyte-stimulating hormone (α-MSH), NN2-0453 and RM493, cause weight loss in the absence of pressor activity, demonstrating the feasibility of separating these two MC4R-mediated activities (Royalty J E, Id.; Kievit P, Id.; Fosgerau K, et al. (2014) Novel alpha-MSH analog causes weight loss in obese rats and minipigs and improves insulin sensitivity. J Endocrinol 220(2):97-107).
In vivo, the activity of the MC4R is regulated by multiple endogenous ligands, including the agonists α, β, and γ-MSH, along with agouti-related protein, (AgRP). It is also regulated by multiple accessory proteins, including attractin-like protein (Haqq A M, et al. (2003) Characterization of a novel binding partner of the melanocortin-4 receptor: attractin-like protein. Biochem J 376(Pt 3):595-605), and MRAP2 (Asai M, et al. (2013) Loss of function of the melanocortin 2 receptor accessory protein 2 is associated with mammalian obesity. Science 341(6143):275-278; Josep-Agulleiro M, et al. (2013) Melanocortin 4 receptor becomes an ACTH receptor by coexpression of melanocortin receptor accessory protein 2. Mol Endocrinol 27(11): 1934-1945; Sebag J A, et al. (2013) Developmental control of the melanocortin-4 receptor by MRAP2 proteins in zebrafish. Science 341(6143):278-281). In the case of most GPCRs, including other melanocortin receptors, loss of function mutations in one of the two alleles typically produces little to no phenotypic effect, perhaps due to the existence of spare receptors (Asano T, et al. (1984) Reconstitution of catecholamine-stimulated binding of guanosine 5′-O-(3-thiotriphosphate) to the stimulatory GTP-binding protein of adenylate cyclase. Biochemistry 23(23):5460-5467). In the case of many GPCRs, only a fraction of the receptors present need to be occupied by ligand to cause a full signaling and physiological effect. In contrast, the MC4R displays a gene-dosage effect, and haploinsufficiency in patients (Cody J D, et al. (1999) Haplosufficiency of the melancortin-4 receptor gene in individuals with deletions of 18q. Hum Genet 105(5):424-427) or mice (Huszar D, et al. (1997) Targeted disruption of the melanocortin-4 receptor results in obesity in mice. Cell 88:131-141) causes a morbid obesity syndrome with intermediate severity between the wild type and homozygous null individuals. MC4R haploinsufficiency causes intermediate effects on nearly all phenotypes measured, including food intake, body weight, linear growth, and autonomic outflow (Huszar D, Id.; Butler A A, et al. (2001) Melanocortin-4 receptor is required for acute homeostatic responses to increased dietary fat. Nature Neuroscience 4(6):605-611; Srisai D, et al. (2011) Characterization of the Hyperphagic Response to Dietary Fat in the MC4R Knockout Mouse. Endocrinology 152(3):890-902; da Silva A A, et al. (2008) Endogenous melanocortin system activity contributes to the elevated arterial pressure in spontaneously hypertensive rats. Hypertension 51(4):884-890). This rheostat-like property of the MC4R suggests that modest increases in MC4R signaling caused by a pharmaceutical agent could have beneficial outcomes on the obesity syndrome of MC4R haploinsufficient patients, and can even be useful in common obesity. What are thus needed are positive allosteric modulators (PAM) of MC4R and uses thereof to provide controlled potentiation of MC4R that respects the spatiotemporal characteristics of the activation of the receptor. Also needed are new methods of treating obesity, especially methods that do not induce the unwanted pressor effect. Still further, there is a need for molecules that can modulate MC4R signaling while allowing the retention of both the spatiotemporal characteristics and the regulation of this pathway by negative feedback loops and AgRP. The compositions and methods disclosed herein address these and other needs.